DTMF signaling has long been used to communicate between telephone terminal equipment and network infrastructure, recording devices, other terminal equipment, and many other kinds of devices. Each key on a typical telephone station keypad dialer, usually corresponding to one of the digits 0 through 9 and star (*) and pound (#) symbols, is used to generate signals with a distinct pair of frequencies (a dual tone) that can be decoded to retrieve keyed digits and symbols. For simplicity, the digits and symbols selectable from a standard telephone keypad will be collectively referred to in this application as DTMF digits unless a distinction is necessary. A primary use of such keypads, DTMF digits, and associated tone pairs, has been to signal telephone digits to a telephone company central office when placing (“dialing”) a telephone call from one of a wide variety of telephone station sets. As is well known, telephone numbers for telephone station sets typically comprise a sequence of seven- or ten-digits, with appropriate prefix and or suffix sequences to select particular services or to select other particular options. Of course, any number of digits can be used in particular calling sessions if mutually understood in the particular network.
Uses of keypads on telephone sets and other DTMF signaling equipment, including communication equipment embedded in network infrastructure, have come to be used for other than telephone station selection (“dialing”). Thus, for example, telephone calling card numbers and Personal Identification Numbers (PINs) are frequently entered using a telephone keypad to signal to remote network infrastructure using DTMF signaling. Telephone, or computer keypads or keyboards, are also used to send DTMF digits for many non-telephone applications, including entering information into Automated Teller Machines (ATMs) or other banking of devices, menu selection at Internet web sites and innumerable others. One important use of DTMF signaling is to indicate assent on the part of a called party to having a received collect call charged to the called telephone station account—popularly, accepting charges for a collect call.
Telephone calls made by inmates in jails, prisons and other custodial institutions are usually processed using so-called Inmate Calling Services (ICSs). A provider of such services may also be called an Inmate Calling Service, but will here be distinguished, when appropriate, by using the term ICS provider. An ICS provider typically employs a variety of processors, databases and other equipment elements individually well known in the art in a system (here referred to as an ICS System or ICS infrastructure) to perform the many functions involved in ICSs. Particular functions, such as call control, three-way call treatment (discussed below) and many others are typically configured and performed under stored program control of such ICS systems.
Custodial institutional regulations typically limit calls from an inmate to only those to specific authorized called parties. Usually, a list or computer database is consulted in response to a call request made by an inmate presenting a called party telephone number to determine whether the requested call is to an authorized called party. Such databases provide a called number validation or screening to prevent harassing calls from being made to, for example, police or other government officials, or to seek to prevent conversations relating to criminal activity, among other reasons. Of course, efforts are made to prevent calls to those who have indicated an unwillingness to receive calls from an inmate caller.
Modern technology has provided ample means for inmates and their accomplices to seek to circumvent call destination (called number) limitations. Thus, for example, so-called three-way (3-way) calls may be attempted by seeking to bridge (or conference) an existing (2-way) first call that has been completed from an inmate to an otherwise authorized called party with a second call from the authorized called party to an unauthorized second called party. In particular cases, the first called party may retain a presence on these bridged first and second calls, or the first called party may hang up the first call. In the latter case, the call from the inmate will effectively have been forwarded to the second unauthorized called party.
Many schemes have been devised by enterprising inmates and their non-inmate collaborators to have a call to an authorized called party routinely forwarded to a third party who, in many cases, may be an unauthorized called party. It should be noted, however, that some three-way or conference calls may be authorized, as in the case where a family member and an attorney for a calling inmate may be permitted. In such cases, however, it is important that the ICS provider, in keeping with institutional regulations, provide control over who may be connected in such authorized conference calls. Incorporated patent application 5 is directed, in part, to techniques for affording ICS providers with improved control over participation in calls involving inmate callers.
Different versions of bridging or conferencing of telephone calls by local and long distance telephone companies provide their subscribers with different particular connections or sets of connections to continue after the completion of the first call. For example, bridging may be accomplished by the called party to the first call using a second telephone line at the station set of that first called party to reach (and bridge or conference) the unauthorized second called party. The terms bridge and conference will be used interchangeably in this application unless a distinction is required by a particular context. It is noted, however, that some practitioners in the telecommunications arts seek to distinguish extending a call to a third party solely through a telephone company central office, or the equivalent, from a bridge established at a (first) called party station. Call forwarding likewise has several different implementations. Some of these implementations, commonly provided as a local (or other) telephone company service, involve behavior, such as keying in a prefix or code (such as 72*) followed by the forwarded-to number. Moreover, such activation of call forwarding on a telephone line (associated with a subscriber number or account) can often be performed remotely from anywhere a DTMF telephone can be used to place a call to the telephone company providing such remote call forwarding service. Typically, call forwarding by a local telephone provider will, once activated by a first code will remain active until deactivated by an authorized person, such as the account owner. For simplicity in the sequel, all calls involving bridging, conferencing or forwarding of calls to unauthorized parties will be referred to as unauthorized 3-way calls, or simply 3-way calls—and the actions seeking to establish or maintain such calls as 3-way calling.
ICS providers use specialized call handling equipment to process a request for a call by an inmate to a called party identified by the inmate by temporarily connecting the calling party to ICS infrastructure equipment—effectively placing the calling party on hold. This on-hold condition persists until the called party is available to be connected (bridged) with a connection from the ICS infrastructure to the called party.
Since many, if not most calls by inmates using an ICS are collect calls, use of DTMF to indicate willingness of a called party to pay for a received call from an inmate is quite common. In typical practice, recorded voice prompts are played when a called party answers a call from an inmate, but before the inmate is bridged to the connection to the authorized called party. This voice prompt will indicate the nature of the call and the identification of the calling party, and will require that the called party depress one or more identified keys on his/her telephone keypad, e.g., “Please press the three key if you agree to have this call charged to your telephone account.” Then, receipt by the ICS provider of the DTMF tones associated with the requested digit or digits, e.g., the “3,” is taken as assent by the called party to have the call charged to his or her account. Generally, if such assent to being charged for the collect call is not received in due course by the ICS provider, the call will be terminated. Alternatively, the voice prompt will be repeated, other voice prompts will be delivered to the called party, or other communication will ensue between the ICS provider and the called party until agreement to pay for the call is received or the call is terminated.
When the called party agrees to have the incoming collect call charged to his/her account, the connection from the calling inmate to ICS infrastructure is bridged to the connection from the ICS provider to the called party. In typical practice, ICS equipment will record and a confinement officer may monitor some or all of: the call request by the inmate, the voice announcements and prompt(s) from the ICS provider to the called party, the called party responses (including DTMF signals received by the ICS provider). As well, as the bridged telephone connection between the calling inmate and the called party that occurs after acceptance of charges by the called party will typically be monitored and/or recorded. Of course, any subsequent bridging or forwarding of connections to include the calling party and other than the original called party will typically be monitored and/or recorded. Though the totality of the call activity for each particular inmate-originated call will typically be recorded, the degree and kind of active monitoring (human and/or machine) may vary during the course of a call. However, an ICS provider actively maintains some presence on each on-going call, and the call participants are made aware of such presence.
Because certain signals appear on a telephone connection involving a station seeking to bridge or conference a third party to an existing call completed from an inmate to an authorized called party, attempts to thwart, or at least detect, unauthorized 3-way calling have tended to focus on detecting signals arising from activities by the authorized called party in attempting to establish a 3-way call. Thus, in particular, monitoring of line signals during an on-going connection for the presence of so-called hook-flash signals used by some telephone companies in providing conferencing or call forwarding has been employed. In other attempts at 3-way call detection, other particular telephone line signal characteristics have been monitored, such as background noise levels, call progress tones, echoes and many others. However, because communications networks are subject to a variable array of noise conditions, echoes and other impairments, and because monitoring equipment and software often prove inadequate to their task of identifying unauthorized calling, and for other reasons, prior 3-way call detection efforts have proven largely unsatisfactory. Further, it has proven costly in terms of processing resources needed for continued monitoring of an on-going call for any of a large number of signal conditions possibly indicative of 3-way call attempts. The incidence of false-positive indications of 3-way call attempts has likewise resulted in the inconvenience of unnecessary call interruptions and terminations, and consequent loss of revenue for ICS providers.
DTMF signal monitoring has been used to identify signal condition involved in establishing a 3-way call, but such DTMF monitoring has typically been in the form of detecting the number of DTMF digits keyed by a called party after an otherwise authorized call has been in progress. Thus, detection of more than a predetermined number DTMF digits has, in some cases, been deemed to be indicative of the placing of an unauthorized 3-way call by an authorized called party. (In confinement institutions it proves relatively simple to prevent the generation of DTMF signals by an inmate by merely disabling the institutional keypad after initial call set-up information has been entered.)
However, because keyed DTMF digits are now being used for a variety of allowable purposes after a call is connected, voice and other signals on the line will be present on a connection while the DTMF detector is active. Indeed, some monitoring or other voice processing applications require a DTMF detector to be capable of recognizing DTMF tones simultaneously with an ongoing voice conversation, while recording a message, playing a message or at other times. For such applications, a DTMF detector must be capable of interpreting received tones in a reliable manner, even when these tones are significantly degraded when compared to DTMF tones transmitted from user equipment to a DTMF detector located in a local telephone central office. If the DTMF detector mistakes speech from an outgoing recording from a voice processing system or a speech signal present on the line as a valid DTMF tone, a monitoring system may enter some undesired mode, resulting in system failure. Mistakes of this type are more common because particular received DTMF tones are not always known in advance or expected, nor is the time of arrival for such tones known.
Importantly, as described in incorporated patent application 7, inadvertent depression of one or more keypad digits can result in a false determination that a 3-way call attempt has been made or is in progress.
It is therefore desired that the ubiquity of DTMF signaling be exploited in 3-way call detection while avoiding shortcomings of prior art 3-way call detection techniques.